The use of high power laser beams to anneal implant damage in semiconductor materials or to achieve crystallization of amorphous thin films has been investigated intensively in the past several years. Some of the advantages of laser annealing over conventional furnace annealing are: fast and room temperature processing without the need of furnaces; laser annealing is inherently localized and a very rapid heating process so that deleterious effects that go with heating the entire semiconductor wafer, for example in a furnace, do not occur; semiconductor wafers do not need capping layers with laser annealing processing and, moreover, often less photolithographic process steps are needed. In addition, significant advantages occur from laser annealing in that one can achieve higher dopant activation than allowed by the solid solubility. Moreover, the dopant profile can be controlled with a higher degree of accuracy. For example, an ion implanted layer can be annealed with little or no impurity redistribution. Accordingly, precise control of the impurity profile in fabricating fine device structures is achievable. Still another advantage of laser annealing is that it can achieve graphoepitaxy which is the crystallization of amorphous or polycrystalline films on texturized amorphous substrates such as silicon oxide (SiO.sub.2 ) or silicon nitride (Si.sub.3 N.sub.4).
There are, in general, three types of known lasers being used in the laser annealing art. They are:
1. Q-switched neodymium:glass (Nd:glass) or ruby laser; PA1 2. Q-switched neodymium:yttrium aluminum garnet (Nd:YAG) laser; and PA1 3. Continuous Wave (CW) lasers.
The Nd:glass laser usually has a pulse width of 10-50 nanoseconds (ns). This type of laser has a relatively large beam spot diameter in the order of 1-2 centimeters (cm) and thus can be used to irradiate a 2 inch (5 cm) wafer with a single pulse. However, such a laser develops hot spots and cannot reproduce the pulse energy reliably.
The Nd:YAG laser has a pulse width in the order of 75-300 ns with a repetition rate of 5 to 10 KHz. This sort of laser is more stable and provides better reproducible beam energies than the Nd:glass laser. However, the small beam spot size of such lasers in the order of 40 .mu.m in diameter discourages its use for annealing semiconductor materials, such as wafers, since the amount of time necessary to achieve a total surface exposure of the beam is long. Moreover, such lasers develop "puddles" of the semiconductor material due to the melting of the material on the surface by the laser beam during the laser annealing process. "Puddles" or "puddle topography" are illustrated in FIGS. 2 and 3 to be later described.
The CW lasers are usually used with a 1-10 millisecond (ms) beam dwell time. The advantage of such lasers is that there is no redistribution of ion-implanted impurities after CW laser annealing. However, the use of CW lasers are too slow for practical manufacturing processes and moreover the CW laser beam induces stress in the annealed surfaces.
There is much in the literature discussing laser annealing, and, more particularly, with respect to the present invention, the process known as "pulsed plasma annealing" (PPA) which should be distinguished from "pulse laser annealing". See the following publications for detailed discussions on various aspects of PPA: "Epitaxial laser crystallization of thin-film amorphous silicon" by J. C. Bean et al., Appl. Phys. Lett., 33(3), August 1978, pp. 227-230; "Time-Resolved Raman Scattering and Transmission Measurements During Pulsed Laser Annealing" by A. Compaan et al. in Laser and Electron-Beam Solid Interactions and Material Processing, pp. 15-22, published by Elsevier North Holland, Inc., 1981; "Reasons To Believe Pulsed Laser Annealing of Si Does Not Involve Simple Thermal Melting" by J. A. Van Vechten et al., Phys. Lett., Dec. 10, 1979, Vol. 74A, No. 6, pp. 417-421; and "Nonthermal Pulsed Laser Annealing of Si; Plasma Annealing" by J. A. Van Vechten et al., Phys. Lett., Dec. 10, 1979, Vol 74A, No. 6, pp. 422-426. Special note is made of the two articles by Van Vechten et al. discussing non-thermal annealing. According to their hypotheses silicon is annealed under certain conditions, by PPA without melting.
See an article by R. A. Kaplan, et al. entitled "Laser Cold Processing takes the heat off Semiconductors," in Electronics, Feb. 28, 1980, pp. 137-142 for a summary of laser systems (Table 3, p. 139) used in the prior art.
None of the above laser systems are capable of annealing wafers having diameters in the order of 3-4 inches (7.5-10 cm) with reasonable uniformity, reproducibility and wafer throughput. To date, experimental use of these equipments for laser annealing has been done to the best of my knowledge only with small batch samples. There appears to be no manufacturing facility or production line that provides for laser annealing in a process schedule even though the advantages of laser annealing, as outlined above, are attractive.